/* Copyright (c) 2007, Jan Flora <janflora@diku.dk>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 *  - Redistributions of source code must retain the above copyright notice, this
 *    list of conditions and the following disclaimer.
 *  - Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 *  - Neither the name of the University of Copenhagen nor the names of its
 *    contributors may be used to endorse or promote products derived from this
 *    software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
 * SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
 * TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
 * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/*
  @author Jan Flora <janflora@diku.dk>
*/

/*
	This module implements a 15 bit linear feedback shift register (LFSR) random number generator.
	the 15 bit length is chosen in order to get a 2 bit maximal length tap sequence.
	The feedback polynomial used is: f(x) = x^15 + x^14 + 1.
	NOTE: This generator must be seeded before the rand command is called.
*/ 

module FastLfsrM
{
	provides interface Random<uint16_t> as Random;
}
implementation
{
	uint16_t shiftReg;
	
/*	async command result_t Random.init()
	{
		// We seed the generator with the last 15 bits of the MAC address.
		return call Random.reseed(((uint16_t*)aExtendedAddress)[0]);
	}*/
	
	async command result_t Random.seed(uint16_t seed)
	{
		// Reseed the LFSR.
		shiftReg = seed & 0x7FFF;
		return SUCCESS;
	}
	
	async command uint16_t Random.rand()
	{
		// Feedback bit equals bit 14 XOR bit 13.
		uint8_t feedback = (((shiftReg>>14)&1) != ((shiftReg>>13)&1));
		// Perform the register shift.
        shiftReg = (shiftReg<<1) + feedback;
		// Eliminate bit 15.
        shiftReg &= 0x7FFF;
	}
}
